Method of Recovering Paraxylene in a Pressure Swing Adsorption Unit with Varying Hydrogen Purge Pressures

ABSTRACT

A method of recovering paraxyiene in a pressure swing adsorption unit with varying hydrogen purge pressures. The pressure swing adsorption zone is adapted to adsorb and desorb paraxyiene based on the cycling of partial pressure in the zone. A first hydrogen purge fed to the zone is within 50 psi of the adsorption pressure of paraxyiene in the zone. A second hydrogen purge fed to the zone is within 50 psi of the desorption pressure of paraxyiene in the zone. The overall amount of hydrogen necessary to operate the pressure swing adsorption zone is reduced and heat may be recovered from the effluent leaving the pressure swing adsorption zone.

The present teachings relate generally to processes for recoveringparaxylene, and in particular, to processes utilizing pressure swingadsorption for recovering paraxylene.

BACKGROUND

Paraxylene is a chemical intermediate that is oxidized to formterephthalic acid, which is a precursor to polyester.

Paraxylene is typically manufactured and recovered from streamscomprising “mixed xylenes.” In the industry, mixed xylenes refer to anarrow boiling distillation heart cut of C8 aromatic hydrocarbonscomprising the three xylene isomers orthoxylene, metaxylene, andparaxylene, as well as the structural isomer ethylbenzene. Mixed xylenesmay also contain non-aromatic compounds with boiling points close to thexylenes. These mainly comprise C9 paraffins and naphthenes. Mixedxylenes generally also contain low levels of toluene and C9 and higheraromatics present due to their imperfect separation in the distillationtowers used to produce the mixed xylenes heart cut. Mixed xylenes aretypically obtained from a reformate of the refinery catalytic reformerunit or another unit used to produced mixed xylenes, such as anon-selective toluene disproportionation (TDP) unit, a selective toluenedisproportionation (STDP) unit, a non-selective or selective toluenealkylation unit, a toluene/aromatic C9-plus transalkylation (TA) unit oran aromatic C9-plus transalkylation unit.

Paraxylene manufacturing units typically have three sections in arecycle loop: 1) a reaction section comprising a xylene isomerizationcatalyst and an ethylbenzene conversion catalyst; and 2) a fractionationsection for separating byproducts produced in the reaction sectionand/or present in the fresh feed; and a 3) a paraxylene recovery sectionfor recovering paraxylene from a mixture of xylene isomers andethylbenzene. A recycle returns a paraxylene-lean stream formed as areject filtrate in the paraxylene recovery section to the reactionsection.

The isomerization catalyst returns a paraxylene-lean stream to its nearequilibrium ratio of 1:2:1 (paraxylene:metaxylene:orthoxylene). Theethylbenzene conversion catalyst is also present because it is notpractical to remove ethylbenzene by distillation because its boilingpoint is very close to the xylene isomers. Thus, ethylbenzene must beconverted to xylenes or to byproducts that can be easily separated bydistillation to prevent its build-up in the loop. For example,ethylbenzene isomerization-type catalysts (also known as naphthene poolcatalysts) have the ability to convert a portion of the ethylbenzene toxylene isomers via C8 naphthene intermediates. Ethylbenzenedealkylation-type catalysts convert ethylbenzene primarily via reactionwith hydrogen to form benzene and ethane. Ethylbenzenetransalkylation-type catalysts convert ethylbenzene primarily by thetransfer of the ethyl group to another ethyl benzene or to a xylene.

All of these catalysts produce by-products from the ethylbenzeneconversion reactions and/or side reactions that must be separated in thefractionation section. These by-products include benzene, toluene, andC9-plus aromatics. The fractionation zone also removes C9-plus aromaticsand other heavies present in the feed.

Two known methods for recovering paraxylene in the paraxylene recoverysection are crystallization and selective adsorption. Selectiveadsorption processes include the UOP Parex process described in R AMeyers (editor) Handbook of Petroleum Refining Processes, Third Edition(2004) and the Axens Eluxyl process described in G Ash, et al, Oil andGas Technology, 49 (5), 541-549 (2004). However, crystallization isoften preferred to selective adsorption because it leads to overallprocess energy savings. Although xylene isomers and ethylbenzene haveundesirably similar boiling points (making distillation difficult), theyhave dramatically different melting points. Pure paraxylene freezes at56° F. (13° C.), pure metaxylene freezes at −54° F. (−48° C.), pureorthoxylene freezes at −13° F. (−25° C.) and pure ethylbenzene freezesat −139° F. (−95° C.).

In a typical crystallization zone for recovering paraxylene, liquidparaxylene is crystallized from a feedstream comprising the xyleneisomers and ethylbenzene. The paraxylene is generally caused tocrystallize by cooling the feedstream to a temperature below thefreezing point of the paraxylene but preferably above the freezing pointof the other components in the feedstream. More particularly, thetemperature is selected to seek to optimize the crystallization ofparaxylene, for example by selecting a temperature at which paraxylenefreezes but which is above the eutectic temperature (the eutectictemperature is the temperature at which a xylene isomer other thanparaxylene begins to co-crystallize). The paraxylene-metaxylene andparaxylene-orthoxylene eutectic temperatures can be close depending onthe composition within the crystallizer, so either metaxylene ororthoxylene may be the first isomer to begin to co-crystallize. Fornon-selective feedstocks, the eutectic temperature is typically around−88° F. (−67° C.) to around −94° F. (−70° C.).

The low temperatures required to crystallize paraxylene from xylenemixtures are typically achieved by a cascaded vapour compressionrefrigerant system using a Deep Refrigerant. A Deep Refrigerant isdefined as one for which it is generally not possible, or not economic,to compress its vapour or gas to a pressure level where it can becondensed by air or water cooling. Ethylene is a Deep Refrigerant,because its critical temperature is 49° F. (9.5° C.), and its criticalpressure is 50.76 bar. Thus, for most places on earth, for at least partof the year, ethylene is a gas above its critical temperature at ambienttemperature, and it is not possible to condense ethylene via air orwater cooling. When used as a refrigerant, ethylene is usually condensedby transferring heat to a High Level Refrigerant. A High LevelRefrigerant is defined as one for which it is possible to condense itsvapour against air or water. Thus, a cascaded ethylene/propylene,ethylene/propane, or ethylene/ammonia refrigeration system can be usedto achieve the low temperatures required for paraxylene crystallization.

Effluent from the crystallization zone contains paraxylene solidsdispersed in a mother liquor, and it will typically therefore benecessary to separate these solids in one or more solid-liquidseparation devices, such as centrifuges. Separation of the effluentproduces a filtrate and a relatively paraxylene-rich cake. The cakeobtained by separating the effluent from the crystallization stagecontains paraxylene crystals with adhered mother liquor that containsethylbenzene, other xylene isomers, unrecovered paraxylene and othercomponents of the feedstream. To improve the purity, the cake istypically further processed in one or more reslurry zones in which thecake is equilibrated with a diluent stream comprising liquid paraxyleneto provide a slurry. The reslurry effluent is separated in asolid-liquid separator to form a relatively pure paraxylene solidproduct and a filtrate that may be recycled or used in other parts ofthe process.

Another method for recovering paraxylene from mixed xylenes is known aspressure swing adsorption and is disclosed, for example, in U.S. Pat.Nos. 6,573,418, 6,600,083, 6,627,783, 6,689,929, and 7,271,305. In apressure swing adsorption unit, a vapor phase containing mixed xylenesis fed at elevated temperature and pressure to a bed of fixed adsorbentcontaining a selective molecular sieve. Paraxylene and ethylbenzene arepreferentially adsorbed to the sieve. The remaining stream is rich inmetaxylene and orthoxylene and passes out of the pressure swingadsorption unit. The pressure is then lowered and paraxylene andethylbenzene are desorbed to form a paraxylene and ethylbenzene richeffluent stream. This effluent may be then sent to a crystallizationzone for recovery of the paraxylene.

Prior pressure swing adsorption units were effective in separating andrecovering paraxylene. However, one of the variable costs of the priormethods was to compress hydrogen for circulation to the pressure swingadsorption unit. There remains a need to develop cost efficientprocesses for manufacturing paraxylene.

SUMMARY

According to one aspect of the invention, a process for the recovery ofa paraxylene product from a mixture of C8 aromatic hydrocarbons isprovided. The process includes feeding a C8-rich aromatic hydrocarbonmixture to a pressure swing adsorption zone to form a paraxylene-richstream and a paraxylene-lean stream. The pressure swing adsorption zoneis adapted to adsorb and desorb paraxylene based on the cycling ofpressure in the zone. The cycling of pressure in the zone uses a firsthydrogen purge being within 50 psi of the adsorption pressure ofparaxylene in the zone and the second hydrogen purge being within 50 psiof the desorption pressure of paraxylene in the zone. The source ofhydrogen supplying the first hydrogen purge is from different from thesource of hydrogen supplying the second hydrogen purge.

Other aspects of the invention will be apparent to those skilled in theart in view of the description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a process flow diagram for manufacturing paraxyleneaccording to one prior art method.

FIG. 1b shows a process flow diagram for manufacturing paraxylene inaccordance with one embodiment of the present invention.

FIG. 2 shows a process flow diagram of a pressure swing adsorption zonein accordance with one embodiment of the present invention.

FIG. 3 shows a process flow diagram of a paraxylene recovery zone inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION

By way of general introduction, a process for recovering paraxylene isdisclosed. The process includes a pressure swing adsorption zone forseparating paraxylene and ethylbenzene from metaxylene and orthoxylene.The present invention reduces the overall amount of hydrogen necessaryto operate the pressure swing adsorption zone. This results in costsavings, particularly in electricity costs associated with the hydrogencompressor. Furthermore, the pressure swing adsorption zone of thepresent invention sets the desorption pressure at a pressure aboveambient, allowing heat to be recovered and thus lowering variable costof the overall process.

Referring now to FIG. 1a and FIG. 1b , a process for the recovery ofparaxylene product is shown generally at 10. FIG. 1a illustrates a priorart process for manufacturing paraxylene, and FIG. 1b illustrates aprocess in accordance with the present invention. A fresh feed 12including mixed xylenes is fed to a fractionation zone including axylene recovery distillation column 20 for separation of C8-richaromatic hydrocarbon mixture from other components. In one embodiment,the mixed xylene fresh feed comprises paraxylene, orthoxylene,metaxylene, as well as the structural isomer ethylbenzene. In otherembodiments, the fresh feed also comprises C7 and C9+ aromaticcompounds, as well as non-aromatic compounds such as C9 paraffins andnaphthenes. Typically, the mixed xylene fresh feed 12 is formed asreformate of a refinery catalytic reformer unit, or another unit used toproduced mixed xylenes, such as a non-selective toluenedisproportionation (TDP) unit, a selective toluene disproportionation(STDP) unit, a non-selective or selective toluene alkylation unit, atoluene/aromatic C9-plus transalkylation (TA) unit or an aromaticC9-plus transalkylation unit. The mixed xylene fresh feed 12 istypically at least 90 wt % mixed xylenes. In some embodiments, the mixedxylene fresh feed 12 is at least 95 wt %, 98 wt %, or 99 wt % mixedxylenes.

In some embodiments, a second fresh feed mixed xylene containing stream14 is also fed the column 20. The second mixed xylene containing stream14 is typically a heavier cut of reformate containing a higherconcentrations of C9+ compounds, and is fed lower on the column 20 thanthe first fresh feed 12. The second mixed xylene stream 14 typicallycontains at least 10 wt % of C9+ compounds. In some embodiments, themixed xylene fresh feed 12 is at least 15 wt %, 20 wt %, 25 wt %, 35 wt%, or 50 wt % of C9+ compounds.

At least one other xylene containing stream resulting from recycle loopsin the process 10 is fed to the column 20. In the embodiment shown, twosuch feeds 16, 18 are shown. Those skilled in the art will appreciatethat other configurations of the recycled feed are also possible. Thexylene containing feed stream 16 and xylene containing feed stream 18typically contain proportionally less ethylbenzene than the mixed xylenefeed streams 12, 14. The feed streams 16 and 18 further comprisebenzene. In some embodiments, the feed streams contain at least 1 wt %or 2 wt % benzene. In other embodiments, feed stream 16 contains atleast 5 wt % benzene.

The xylene recovery column 20 is configured to separate the feed streams12, 14, 16, 18 into one or more streams comprising a C8-rich aromatichydrocarbon mixture, a stream containing C7− compounds, and a streamcontaining C9+ compounds. In the embodiment shown in FIG. 1a or FIG. 1b, a first sidedraw stream 22 comprises a liquid phase C8-rich aromatichydrocarbon mixture, while a second sidedraw stream 24 comprising avapor phase C8-rich aromatic hydrocarbon mixture. The first sidedrawstream 22 is withdrawn at location on the column above the secondsidedraw stream 24. The feed stream 18 is introduced to the column abovethe vapor phase sidedraw stream 24 so that gaseous components in thefeed stream 18 do not exit through the vapor phase sidedraw stream 24.The liquid phase sidedraw phase is pressurized by pump 23. The vaporphase sidedraw stream is condensed by condenser 26 and the resultingcondensate is pressured by pump 27. The pressurized condensate of thevapor phase sidedraw stream and the pressurized liquid phase sidedrawstream are combined to form a combined C8-rich aromatic hydrocarbonmixture stream 28.

An overhead product stream 30 is withdrawn from the top of the column 20and comprises C7− compounds including benzene, toluene, and ethane. Theoverhead product stream 30 is partially condensed by condenser 32 andthe condenser effluent is separated into liquid and gaseous componentsin flash drum 34. The liquid phase is partially returned to the column30 as a reflux stream 36 b and partially removed from the process viastream 36 a. The gaseous components are removed from the process as alight co-product stream 38.

A bottoms product stream 40 is removed from the bottom of the column 20and comprises C9+ compounds including trimethylbenzene andmethylethylbenzene. A portion of the bottoms product is recovered as abottoms co-product stream 42, while another portion 44 of the bottomsproduct stream 40 is reboiled by reboiler furnace 46 and returned to thecolumn 20. The reboiler furnace 46 provides for the elevated temperatureof the column 20 which operates in a temperature gradient, for example,between 500° F. (260° C.) and 50° F. (10° C.) and a pressure of 15-80psia.

In the prior art process shown in FIG. 1a , the combined C8-richaromatic hydrocarbon mixture stream 28 is fed to a paraxylene recoveryzone 72. However, in the present invention illustrated in FIG. 1b , atleast a portion of the C8-rich aromatic hydrocarbon mixture 28 recoveredfrom the fractionation zone is pre-heated by furnace 50 and one or moreheat exchangers (not shown) and delivered to a pressure swing adsorptionzone 52. In the pressure swing adsorption zone 52, the C8-rich aromatichydrocarbon mixture is fed at elevated temperature and pressure to a bedof fixed adsorbent containing a selective molecular sieve. Paraxyleneand ethylbenzene are preferentially adsorbed to the sieve. The remainingstream is rich in metaxylene and orthoxylene and passes out of thepressure swing adsorption unit as paraxylene-lean stream 54. The partialpressure is then lowered and paraxylene and ethylbenzene are desorbed toform a paraxylene-rich and ethylbenzene-rich effluent stream 56. Theconfiguration and operation of the pressure swing adsorption zone ismore fully described below and in reference to FIG. 2.

A first source of pressurized hydrogen purge gas 58 is fed to thepressure swing adsorption zone 52. A second source of hydrogen purge gas60 is formed condensing the paraxylene-rich and ethylbenzene-richeffluent stream 56 in condenser 57 and then flashing in drum 62 toremove hydrogen. The resulting hydrogen-rich stream 64 is compressed bycompressor 66 and the resulting pressured hydrogen-rich stream is asecond source of hydrogen purge gas 60 that is fed to the pressure swingadsorption unit 52. In one embodiment, the first hydrogen purge gas 58is at a higher pressure than the second hydrogen purge gas 60. In oneembodiment, the first hydrogen purge gas 58 is introduced at a pressurebetween 200 and 400 psia, and the second hydrogen purge is introduced ata pressure between 40 and 100 psia. In another embodiment, the firsthydrogen purge is within 50 psi of the adsorption pressure of theparaxylene in the zone and the second hydrogen purge is within 50 psi ofthe desorption pressure of the paraxylene. Typically, the adsorptionpressure will be in the range of 175 psia to 375 psia and the desorptionpressure will be in the range of 30 psia to 90 psia. By maintaining adesorption pressure above ambient, the paraxylene-rich andethylbenzene-rich effluent 56 may be maintained at temperature highenough to allow useful amounts of heat to be recovered in condenser 57.In one embodiment, the temperature of the effluent 56 entering thecondenser 57 is between 150° F. (65.6° C.) and 400° F. (204.4° C.).

The paraxylene-rich and ethylbenzene-rich effluent 70 exiting the flashdrum 62 is fed to a paraxylene recovery zone 72. In one embodiment, aC8-rich aromatic hydrocarbon mixture feed 74 to the paraxylene recoveryzone 72 comprises a second portion of the combined C8-rich aromatichydrocarbon mixture stream 28 exiting the column 20 and bypasses thepressure swing adsorption unit 52. In one embodiment, the second feed 74comprises at least 10 wt % of the combined paraxylene-rich andethylbenzene-rich stream 28. In other embodiments, the second feed 74comprises at least 20 wt %, at least 30 wt %, at least 40 wt %, at least50 wt %, at least 60 wt %, at least 70 wt %, at least 80 wt %, or atleast 90 wt % of the combined C8-rich aromatic hydrocarbon mixturestream 28.

The paraxylene recovery zone 72 operates to produce a paraxylene product76 and to recycle a paraxylene-lean stream 78 for further processing. Inone embodiment, the paraxylene recovery zone 72 is configured to recoverparaxylene product through a selective adsorption process. In anotherembodiment, the paraxylene recovery zone 72 is configured is configuredas to recover paraxylene through a crystallization process. Oneparticular crystallization process is described below in reference toFIG. 3.

The paraxylene-lean stream 54 exiting the pressure swing adsorption unit52 is fed to a isomerization reactor 80. The isomerization reactor 80 isa packed bed reactor containing a bed of an isomerization catalyst forconverting metaxylene and orthoxylene to paraxylene at an approximatelyequilibrium ratio of 1:2:1 (paraxylene:metaxylene:orthoxylene). In oneembodiment, hydrogen 82 is added to the paraxylene-lean stream 54upstream of the isomerization reactor 80. In another embodiment, theparaxylene-lean 54 contains enough hydrogen after exiting the pressureswing adsorption unit 52 that make-up hydrogen is not added to the feed54.

The second paraxylene-lean stream 78 exiting the paraxylene recoveryzone 72 is mixed with fresh hydrogen 84 and pre-heated with a furnace 86and/or one or more heat exchangers (not shown). The preheated mixture 88is fed to the additional isomerization reactor 90. The isomerizationreactor 90 contains a isomerization catalyst for converting metaxyleneand orthoxylene to paraxylene at an approximately equilibrium ratio of1:2:1 (paraxylene:metaxylene:orthoxylene). In some embodiments, theisomerization reactor 90 also contains an ethylbenzene conversion,catalyst such as dealkylation catalyst for converting ethylbenzene tobenzene and ethane. Suitable isomerization catalysts and ethylbenzenecatalysts are disclosed, for example, in U.S. Pat. Nos. Re 31,782,4,899,011, and 6,518,472.

In the embodiment shown, the isomerate stream 91 from the isomerizationreactor 80 and the isomerate stream 92 from the additional isomerizationreactor 90 are fed to a high temperature separator 94 where the streamsare mixed and flashed. A liquid-rich phase bottom stream from the hightemperature separator 94 is one of the xylene containing feed stream 18to the column 20. A vapor-rich phase stream 95 exiting high temperatureseparator 94 is sent to a low temperature separator 96 where thevapor-rich phase stream is flashed. A liquid-rich phase stream exitingthe low temperature separator 96 is another of the xylene containingfeed stream 16 fed to the column. A vapor phase stream 98 exiting thelow temperature separator 96 comprises hydrogen, ethane, and other lightcomponents and may be recycled and used as a source for streams 84and/or 58 or be used for fuel.

FIG. 2 shows one embodiment of the pressure swing adsorption zone 52according to the present invention. The pressure swing adsorption zone52 comprises one or more vessels 100 a, 100 b, 100 c, 100 d, 100 e, 100f containing a paraxylene selective adsorbent. In the embodiment shown,there are six vessels, but those skilled in the art will recognize thatother configurations are also possible, such as configurations withdifferent numbers of vessels, e.g. 3 vessels, 4 vessels, 5 vessels, 7vessels, 8 vessels, etc. In one embodiment, the paraxylene selectiveadsorbent is a non-acidic, medium pore, molecular sieve. In oneembodiment, the molecular sieve is of the MFI structure type and theprocess is operated in the vapor phase at elevated temperatures andpressures wherein the temperature is substantially isothermal.Adsorbents useful in the present invention are based on molecular sievesthat selectively adsorb paraxylene within the channels and pores of themolecular sieve while not effectively adsorbing metaxylene andorthoxylene C₈ isomers (i.e., total exclusion of the larger metaxyleneand orthoxylene or having much slower adsorption rates compared toparaxylene).

Molecular sieves are ordered porous crystalline materials, typicallyformed from silica, alumina, and phosphorus oxide (PO₄) tetrahedra, thatcontain a crystalline structure with cavities interconnected bychannels. The cavities and channels within the crystalline structure areuniform in size and may permit selective separation of hydrocarbonsbased upon molecular dimensions. Generally, the term “molecular sieve”includes a wide variety of natural and synthetic crystalline porousmaterials which typically are based on silica tetrahedra in combinationwith other tetrahedral oxide materials such as aluminum, boron,titanium, iron, gallium, and the like. In these structures networks ofsilicon and elements such as aluminum are cross-linked through sharingof oxygen atoms. Substitution of elements such as aluminum or boron forsilicon in the molecular sieve structure produces a negative frameworkcharge which must be balanced with positive ions such as alkali metal,alkaline earth metal, ammonium or hydrogen. Molecular sieve structuresalso may be formed based on phosphates in combination with othertetrahedrally substituted elements such as aluminum.

Adsorbents useful in this invention should not possess catalyticisomerization or conversion activity with respect to the C₈ aromaticfeedstream. Thus, suitable molecular sieves should be non-acidic. If anelement such as aluminum or gallium is substituted in the molecularsieve framework, the sieve should be exchanged with a non-acidiccounter-ion, such as sodium, to create a non-acidic sieve adsorbent.

Examples of molecular sieves suitable as adsorbents useful in thisinvention include zeolitic materials containing pore dimensions in therange of 5 to 6 angstroms (10⁻⁸ meter), typically 5.1 to 5.7 angstroms,and preferably 5.3 to 5.6 angstroms, as measured in cross axes of thepore. This range typically is referred to as “medium pore” and typicallycontains 10-ring tetrahedra structures. Typical examples of medium poremolecular sieves include those with MFI and MEL framework structures asclassified in Meier and Olson, “Atlas of Zeolite Structure Types,”International Zeolite Association (1987), incorporated herein byreference in its entirety. A small pore molecular sieve, such as Azeolite, which contains 8-ring structures does not have a sufficientlylarge pore opening to effectively adsorb para-xylene within the sieve.Most large pore molecular sieves, such as mordenite, Beta, LTL, or Yzeolite, that contain 12-ring structures do not adsorb para-xyleneselectively with respect to ortho- and meta-xylenes. However, several 12ring structures, having a smaller effective pore size, for example dueto puckering, are potentially useful in the invention, such as structuretypes MTW (e.g., ZSM-12) and ATO (e.g., ALPO-31).

Specific examples of molecular sieves include ZSM-5 (MFI structure type)and ZSM-11 (MEL structure type) and related isotypic structures. Sincesuitable adsorbents should not be catalytically reactive to componentsin the feedstream, the preferable adsorbent useful in this invention issilicalite (MFI structure type), an essentially all silica molecularsieve, which contains minimal amounts of aluminum or other substitutedelements. Typically, the silica/alumina ratio of suitable silicalite isabove 200 and may range above 1000 depending on the contaminant level ofaluminum used in the sieve's preparation. Other MFI and MEL sieves maybe use to the extent they are made non-catalytically active. Otherpotentially useful adsorbents include structure types MTU, FER EUO, MFS,TON, AEL, ATO, NES, and others with similar pore sizes.

A molecular sieve which is not catalytically reactive will typicallyexhibit less than 10% conversion of paraxylene to metaxylene andorthoxylene, and in some embodiments, less than 5%, and in otherembodiments less than 1%, at the temperature of operation for theprocess of the invention.

The C8-rich aromatic hydrocarbon mixture enters pressure swingadsorption zone 52 through xylene header 102 and is introduced intovessels 100 a, 100 b, 100 c, 100 d, 100 e, 100 f through respective feedcontrol valves 102 a, 102 b, 102 c, 102 d, 102 e, 102 f. The firsthydrogen purge 58 enters the vessels 100 a, 100 b, 100 c, 100 d, 100 e,100 f through high pressure hydrogen header 106 and high pressurehydrogen feed control valves 106 a, 106 b, 106 c, 106 d, 106 e, 106 f,respectively. The second hydrogen purge gas 60 enters the vessels 100 a,100 b, 100 c, 100 d, 100 e, 100 f through low pressure hydrogen header104 and low pressure hydrogen feed control valves 104 a, 104 b, 104 c,104 d, 104 e, 104 f, respectively.

The pressure swing adsorption zone 52 also comprises a paraxylene andethylbenzene collection header 108 and a set of outlet control valves108 a, 108 b, 108 c, 108 d, 108 e, 108 f for removing a paraxylene-richand ethylbenzene-rich stream 56 from each of the vessels 100 a, 100 b,100 c, 100 d, 100 e, 100 f, respectively. The pressure swing adsorptionzone 52 also comprises a metaxylene and orthoxylene collection header110 and a set of outlet control valves 110 a, 110 b, 110 c, 110 d, 110e, 110 f for removing the first paraxylene-lean stream 54 from thevessels 100 a, 100 b, 100 c, 100 d, 100 e, 100 f, respectively. Thepressure swing adsorption zone 52 also comprises a pressure equalizationheader 112 and a set of equalization control valves 112 a, 112 b, 112 c,112 d, 112 e, 112 f for equalizing the pressure between two or more ofthe vessels 100 a, 100 b, 100 c, 100 d, 100 e, 100 f, respectively. Thepressure swing adsorption zone 52 also comprises a hydrogenpressurization header 114 which is fed from high pressure hydrogensource 106 and a set of pressurization control valves 114 a, 114 b, 114c, 114 d, 114 e, 114 f for pressurizing vessels 100 a, 100 b, 100 c, 100d, 100 e, 100 f respectively.

The vessels 100 a, 100 b, 100 c, 100 d, 100 e, 100 f in the pressureswing adsorption zone 52 are operated in a sequence of operations, thesequence of operations in each vessel being offset in time from thesequence of operations in the other vessels such that the vesselsoperate together in a pseudo-continuous manner.

The sequence of operations are now described with reference to the firstvessel 100 a. All the valves are controlled automatically by a controlsystem (not shown). The valves are maintained closed unless they aredescribed as being opened below for a particular operation.

In the first operation, designated “FEED”, C8-rich aromatic hydrocarbonstream is introduced through feed header 102 and feed control valve 102a to vessel 100 a at elevated pressure. The paraxylene and ethylbenzenemolecules adsorb to the adsorbent, while the metaxylene and orthoxylenemolecules are blown through the bed and leave the process through theoutlet control valve 110 a and the metaxylene and orthoxylene collectionheader 110.

In the second operation, designated “HPPu,” (high pressure purge) thehigh pressure purge gas 58 (FIG. 1b ) is fed through header 106 andcontrol valve 106 a to sweep the bed. This hydrogen displaces all of themetaxylene and orthoxylene left in the void space of the bed andcontinues to flow out through control valve 110 a and header 110 afterthe FEED operation is complete. This allows the paraxylene andethylbenzene to be extracted in a later operation without beingcontaminated by metaxylene and orthoxylene.

In the third operation, designated “E1d”, (equalization #1 down) thepressure in vessel 100 a is equalized with the pressure in anothervessel, for example, vessel 100 d, by opening valves 112 a and 112 d.This depressurization in vessel 100 a is performed so that theparaxylene and orthoxylene can be removed, but in order to keep thehydrogen from being blown out with the paraxylene and ethylbenzene, thepressure is equalized with another vessel that is at the point in theprocess where it needs to start re-pressurizing to get ready for itsnext feed step. This saves hydrogen from being sent out of the processunnecessarily and reduces the overall hydrogen needs.

In the fourth operation, designated “E2d” (equalization #2 down), thepressure in vessel 102 a is subjected to another hydrogen equalizationstep which takes place at a lower pressure than E1d and saves morehydrogen. For example, the pressure in vessel 102 a may be equalizedwith the pressure in vessel 102 c by opening control valve 112 a andvalve 112 c.

In the fifth operation, designated “CnD” (Countercurrentdepressurization), the bed is fully depressurized to its lowestdesorption pressure and the paraxylene and ethylbenzene begin to desorboff the adsorbent and are removed through valve 108 a.

In the sixth operation, designated “LPPU” (Low Pressure Purge), the lowpressure hydrogen 60 (FIG. 1b ) is fed through low pressure hydrogenheader 104 and valve 104 a into vessel 100 a at the desorption pressureand sweeps the bed. This sweeping of the bed further drops the partialpressure of paraxylene and ethylbenzene. This causes further desorptionof paraxylene and ethylbenzene from the adsorbent. This increases thecapacity of the adsorbent such that a commercially relevant amount ofparaxylene and ethylbenzene can be routed through the bed during eachcycle. Without this sweep flow, the capacity would lower and the spacevelocities would be higher.

In the seventh operation, “E2u” (equalization #2 up), hydrogen pressureis equalized with that from another vessel, such as vessel 100 e, whichis simultaneously undergoing E2d, by opening valves 112 a and 112 e.

In the eighth operation, “E1u” (equalization #1 up), hydrogen isequalized with that from another vessel, such as vessel 100 d, which issimultaneously undergoing E1d, by opening valve 112 a and 112 d.

In the ninth operation, “H2P” (hydrogen pressurization), the bed isbrought up to the feed pressure after the two equalization up steps byfresh hydrogen by opening valve 114 a.

All six vessels 100 a, 100 b, 100 c, 100 d, 100 e, 100 f go through thissame cycle of nine operations, but at any given time, each vessel is ata different stage of the cycle. The system is designed and operated suchthat one of vessels is always in the FEED operation so that the feed tothe pressure swing adsorption zone 52 as a whole is constant.

Table 1 illustrates one embodiment of a sequence of the nine operationsfor the pressure swing adsorption zone 52 in which plurality of vesselsoperate together in a pseudo-continuous manner. The Table illustratestwelve time periods and shows which operation is being performed in eachvessel at each time period. A typical time period is from 5 seconds toabout 120 seconds. Those skilled in the art will recognize that thesequence in Table 1 is exemplary and other sequences are also possibleto carry out the invention.

TABLE 1 Sequence of Pressure Swing Adsorption Operations Time PeriodVessel a Vessel b Vessel c Vessel d Vessel e Vessel f 1 Feed HPPu E1dCnD LPPu E1u 2 E2d LPPu E2u H2P 3 HPPu E1d CnD E1u Feed 4 E2d LPPu E2uH2P 5 E1d CnD E1u Feed HPPu 6 E2d LPPu E2u H2P 7 CnD E1u Feed HPPu E1d 8LPPu E2u H2P E2d 9 E1u Feed HPPu E1d CnD 10 E2u H2P E2d LPPu 11 E1u FeedHPPu E1d CnD 12 H2P E2d LPPu E2u

FIG. 3 illustrates one embodiment of the paraxylene recovery zone 72 inaccordance with the present invention. The paraxylene-rich andorthoxylene-rich effluent 70 enters a crystallization zone comprising afirst crystallization zone comprising a first crystallizer 202 and asecond crystallization zone comprising a second crystallizer 216. In oneembodiment, the effluent 70 comprises at least 25 wt % paraxylene. Inother embodiments, the effluent 70 comprises less than 75 wt %, lessthan 85 wt %, or less than 95 wt % paraxylene. The first crystallizer202 is temperature controlled to operate to solidify paraxylene in theeffluent 70. In one embodiment, the first crystallization zone operatesat a temperature greater than −40° F. (−40° C.). In another embodiment,the first crystallization zone operates at a temperature greater than−30° F. (−34.4° C.). The first crystallization zone typically operates atemperature between 40° F. (4.4° C.) and −40° F. (−40° C.). The firstcrystallizer effluent 204 is withdrawn from the first crystallizer 202and sent to a first solid-liquid separator 206. The first solid-liquidseparator 206 separates a paraxylene-lean filtrate stream 208 from afirst paraxylene-rich cake stream 210. One portion 212 of theparaxylene-lean filtrate stream may be recycled to the firstcrystallizer 202, while another portion 214 of the paraxylene-leanfiltrate stream is sent to a second crystallizer 216. The secondcrystallizer 216 is temperature controlled to further solidify anyremaining paraxylene. The second crystallization zone operates at atemperature less than the first crystallization zone. In one embodiment,the second crystallization zone operates at a temperature less than −70°F. (−56.7° C.). In another embodiment, the second crystallization zoneoperates at a temperature less than −90° F. (−67.8° C.). In anotherembodiment, the second crystallization zone operates at a temperatureless than −110° F. (−78.9° C.). The second crystallization zonetypically operates at a temperature between −30° F. (−34.4° C.) and−130° F. (−90° C.). The second crystallizer effluent 218 is withdrawnfrom second crystallizer 216 and introduced to a second solid-liquidseparator 220. In one embodiment, the effluent 218 exiting the secondcrystallizer is colder than −95° F. (−70.6° C.). The second solid-liquidseparator 220 separates a second paraxylene-rich cake 222 from a secondparaxylene-lean filtrate stream 224. A portion 228 of the filtratestream 224 may be recycled to the crystallizer 216, while anotherportion 78 of the paraxylene-lean filtrate stream 224 is recycled forfurther processing as described above and in reference to FIG. 1 b.

The first and second solid-liquid separator 202, 216 may be anysolid-liquid separation devices known in the art, such as centrifuges,rotary pressure filters, rotary vacuum filters, or filter columns. Inone particular embodiment, the first solid-liquid separator 206comprises a pusher centrifuge and the second solid-liquid separator 220comprises a screen bowl centrifuge. In one embodiment, the secondsolid-liquid separator 220 removes an additional paraxylene-leanfiltrate 226 before withdrawing the second paraxylene-lean filtrate 224.The additional filtrate 226 is higher in paraxylene concentration thanthe second paraxylene-lean filtrate 224 and is recycled to the secondcrystallizer 216.

The first paraxylene-rich cake 210 and the second paraxylene-rich cake222 enters one or more reslurrying zones for removing any remainingimpurities. The embodiment of FIG. 3 shows two reslurrying zones, eachhaving a reslurry drum 224, 240. The paraxylene-rich cake 210 from thefirst solid-liquid separator is fed to either or both of the firstreslurry drum 224 and the second reslurry drum 240 through streams 226and 242, respectively. The second paraxylene-rich cake 222 is fed to thefirst reslurry drum 224. The paraxylene-rich cake(s) are reslurried inthe first reslurry drum 224 with reslurrying fluids to remove impuritiesfrom the paraxylene crystals and the effluent 231 from the firstreslurry drum 224 is sent to the third solid-liquid separator 232. Thethird solid-liquid separator 232 separates the effluent 231 into a thirdparaxylene-rich cake 238 and a third paraxylene-lean filtrate stream234. A portion 228 of the third paraxylene-lean filtrate stream 234 isrecycled to the first reslurry drum 224 as a reslurrying fluid, andanother portion 236 may be recycled to the first crystallizer 202 forfurther recovery of paraxylene.

The third paraxylene-rich cake 238 is fed to a second reslurry drum 240for further reslurrying with one or more reslurrying fluids for removingimpurities from the paraxylene crystals. The effluent 248 from thesecond reslurry drum 240 is fed to a fourth solid-liquid separator 250.The fourth solid-liquid separator 250 separates the effluent 248 into afourth paraxylene-rich cake 252 and a fourth paraxylene-lean filtratestream 254. A portion 244 of the fourth paraxylene-lean filtrate stream254 is recycled to the second reslurry drum 244 as a reslurrying fluid,and another portion 230 of the fourth paraxylene-lean filtrate stream254 may be recycled to the first reslurry drum 224 for use as areslurrying fluid.

The third and fourth solid-liquid separator 232, 250 may be anysolid-liquid separation devices known in the art, such as centrifuges,rotary pressure filters, rotary vacuum filters, or filter columns. Thefourth solid-liquid separator 250 may also be a wash column. Suitablefilter columns are disclosed, for example, in U.S. Pat. Nos. 7,812,206,8,211,319, and 8,530,716, and 8,962,906, Suitable wash columns aredisclosed, for example, in U.S. Pat. Nos. 4,734,102 and 4,735,781. Inone particular embodiment, the third solid-liquid separator 232comprises a pusher centrifuge and the fourth solid-liquid separator 250comprises a pusher centrifuge. In one embodiment, the fourthsolid-liquid separator 250 removes an additional paraxylene-leanfiltrate 246 before withdrawing the fourth paraxylene-lean filtrate 254.The additional filtrate 246 is higher in paraxylene concentration thanthe fourth paraxylene-lean filtrate 254 and is recycled to the secondreslurry drum 240.

The fourth paraxylene-rich cake 252 is fed to a melt drum 256. Thefourth paraxylene-rich cake is completely melted and a paraxyleneproduct stream 76 is recovered. A portion 258 of the melted paraxylenemay be recycled to the fourth solid-liquid separator 250 in order towash impurities from the cake. In one embodiment, the paraxylene product76 is at least 99 wt % paraxylene. In other embodiment, the paraxyleneproduct is at least 99.5 wt %, 99.6 wt %, 99.7 wt %, or 99.8 wt %paraxylene.

The use of a pressure swing adsorption zone with an additionalisomerization zone allows for less total mass being fed to theparaxylene recovery zone, because a significant portion of themetaxylene and orthoxylene in the system is recycled through stream 54(FIG. 1b ). In one embodiment, the ratio of the total mass of theparaxylene-rich stream entering the paraxylene recovery zone to thetotal mass of the paraxylene-rich product stream is less than 6. Inother embodiments, the ratio of the total mass of the paraxylene-richstream entering the paraxylene recovery zone to the total mass of theparaxylene-rich product stream is less than 5, less than 4, less than 3,or less than 2. In other embodiments, the ratio of the total mass of therecycle stream 78 (FIG. 1b ) to the total mass of the paraxylene-richproduct stream 76 is less than 5, less than 3, or less than 2. The feedto the paraxylene recovery zone also contains a higher concentration ofparaxylene compared to systems not having a pressure swing adsorptionzone. This is because the pressure swing adsorption zone allows forparaxylene concentrations greater than the equilibrium concentrationresulting from the isomerization reaction.

According to another aspect of the invention, a method for retrofittinga system for recovering paraxylene is provided. According to theretrofitting method, the pressure swing adsorption zone 52 (FIG. 1b ) isadded to a pre-existing system (FIG. 1a ) not having a pressure swingadsorption zone. At least a first portion 51 of the combined C8-richaromatic hydrocarbon mixture stream 28 is routed to the pressure swingadsorption zone 52 to form a paraxylene-rich intermediate stream 56(which is flashed in drum 62 to form stream 70) before being fed to theparaxylene recovery zone 72. The retrofit method may also compriseadding the secondary isomerization zone 80 to a pre-existing systemwhere there was no previous secondary isomerization zone 80. Theretrofit method may also include adding the bypass stream 74 so that asecond portion of the combined C8-rich aromatic hydrocarbon mixturestream 28 routes directly to the paraxylene recovery zone 72, bypassingthe pressure swing adsorption zone 52. The amount of C8-rich aromatichydrocarbon mixture stream bypassed through bypass stream 74 isdependent upon the throughputs of the pressure swing adsorption zone 52and the pre-existing equipment. In one embodiment, the pre-existingequipment does not have to be re-sized as a result of the retrofit,which allows increased recovery of paraxylene without significantcapital expenditures. By enriching the combined stream 28 in paraxyleneprior to its delivery to the paraxylene recovery zone and addingisomerization capacity, the retrofit method allows for increasedrecovery of paraxylene product compared to the pre-existing system. Inone embodiment, the amount of a paraxylene product recovered by theretrofitted system increases without increasing the throughput of theprimary isomerization zone 90. In another embodiment, the amount ofparaxylene product recovered increases without increasing the amount ofhydrogen fed to the system. In another embodiment, the amount ofparaxylene product recovered increases without increasing the amount ofthe refrigeration duty of the crystallization zone. In anotherembodiment, the amount of paraxylene product recovered increases withoutincreasing the amount of the furnace duty 86 of the primaryisomerization zone. In another embodiment, the amount of paraxyleneproduct recovered increases without increasing the amount of the furnaceduty 46 of the fractionation zone.

The foregoing detailed description and the accompanying drawings havebeen provided by way of explanation and illustration, and are notintended to limit the scope of the appended claims. Many variations inthe presently preferred embodiments illustrated herein will be apparentto one of ordinary skill in the art, and remain within the scope of theappended claims and their equivalents.

It is to be understood that the elements and features recited in theappended claims may be combined in different ways to produce new claimsthat likewise fall within the scope of the present invention. Thus,whereas the dependent claims appended below depend from only a singleindependent or dependent claim, it is to be understood that thesedependent claims can, alternatively, be made to depend in thealternative from any preceding claim—whether independent ordependent—and that such new combinations are to be understood as forminga part of the present specification.

1. A process for the recovery of a paraxylene product from a mixture ofC8 aromatic hydrocarbons, the process comprising: feeding a C8-richaromatic hydrocarbon mixture to a pressure swing adsorption zone to forma paraxylene-rich stream and a paraxylene-lean stream, the pressureswing adsorption zone adapted to adsorb and desorb paraxylene based onthe cycling of partial pressure in the zone, the cycling of partialpressure in the zone using a first hydrogen purge being within 50 psi ofthe adsorption pressure of paraxylene in the zone and a second hydrogenpurge being within 50 psi of the desorption pressure of paraxylene inthe zone, the source of hydrogen supplying the first hydrogen purgebeing different from the source of hydrogen supplying the secondhydrogen purge.
 2. The process of claim 1, wherein a pressure of thefirst hydrogen purge is higher than a pressure of the second hydrogenpurge.
 3. The process of claim 1, wherein the source of hydrogensupplying the second hydrogen purge comprises hydrogen separated fromthe paraxylene-rich stream.
 4. The process of claim 1, wherein the firsthydrogen purge is introduced at a pressure between 200 and 400 psia. 5.The process of claim 1, wherein the second hydrogen purge is introducedat a pressure between 40 and 100 psia.
 6. The process of claim 1,further comprising isomerizing the paraxylene-lean stream to form anisomerate having a paraxylene concentration greater than the paraxyleneconcentration of the paraxylene-lean stream.
 7. The process of claim 1,further comprising crystallizing the paraxylene-rich stream to form aparaxylene product stream and a 2^(nd) paraxylene-lean stream.
 8. Theprocess of claim 7, further comprising isomerizing the 2ndparaxylene-lean stream in a 2^(nd) isomerization zone to form a 2^(nd)isomerate having a paraxylene concentration greater than the paraxyleneconcentration of the 2^(nd) paraxylene-lean stream.
 9. The process ofclaim 8, further comprising converting ethylbenzene to other compoundsin the 2^(nd) isomerization zone.
 10. The process of claim 9, furthercomprising feeding the isomerate, the 2^(nd) isomerate, and a mixedxylene feed stream comprising C7, C8, and C9+ aromatic hydrocarbons to afractionation zone, and separating the C8 aromatic hydrocarbons fromother components in the fractionation zone to form said C8-rich aromatichydrocarbon mixture.
 11. The process of claim 1, wherein the pressureswing adsorption zone comprises a plurality of adsorption vessels eachcontaining an adsorbent, each vessel being controlled in a sequence ofbatch operations comprising the first hydrogen purge and the secondhydrogen purge, the sequence of operations each vessel being offset intime from the sequence of operations of the other vessels such that saidplurality of vessels operate together in a pseudo-continuous manner.